Line trimmers are used to cut vegetation such as grass or weeds. Trimming with a line trimmer offers advantages in that the line can cut vegetation against walls or around stones without damage. However, the line normally is abraded when vegetation is cut or when other objects are contacted. The line may also break off close to the housing if the trimmer abruptly contacts the ground or another object. When the line breaks off, it is necessary to feed a new increment of line from the spool, and out of the housing.
Various automatic feed mechanisms have been contrived that have achieved the objective of providing an automatic feed mechanism. However, several problems are inherent in many of the designs. One problem is controlling the length of line allowed to feed from the spool. Another problem is providing an automatic line feed which is easy to use.
A line feed mechanism must be durable. Automatic line feed mechanisms must be capable of enduring wear and tear of high speed operation, as well as impacts with objects. For this reason, line trimmer heads having automatic feed mechanisms requiring close calibration or relying upon small spring elements and multiple piece linkages present problems.
Another aspect of automatic feed line trimmer heads is that they should be easy to manufacture. Line trimmer heads which require intricate assembly or calibration complicate the manufacturing process.
The line feed mechanism must be well suited to provide limited line advance when the line is abraded and becomes worn. It is also desirable to replenish the line immediately upon breakage of the line.
The most common type of automatic line feed advance is commonly referred to as "bump feed" wherein the head of the line cutter is bumped against the ground to cause the spool to feed out a length of line. Examples of bump feed devices include U.S. Pat. No. 4,185,381 to Palmieri, et al.; U.S. Pat. No. 4,274,201 to Oberg, et al.; and U.S. Pat. No. 4,882,843 to Baba. Bump feed line advance mechanisms have tended to be difficult to control. Constantly bumping the head of the unit on the ground to allow line to be advanced increases wear and tear on the device.
A different approach to automatic line feed is to provide a bearing device which is shifted by centrifugal force in response to change in the speed of rotation of the casing. The use of ball bearing members in the head offers advantages in terms of eliminating the need to bump the line trimmer head on the ground to advance line but tends to complicate assembly of the device both initially and when the line spool must be replaced. When the line spool is removed in the field, there is a risk that the ball bearing members may be dislodged from the unit and lost. Examples of this approach include U.S. Pat. No. 4,566,189 to Muto and U.S. Pat. No. 4,667,410 to Weid, et al.
Another approach to an automatic line feed is to provide spring loaded lugs or cams which move into and out of engagement with locking elements. One example is U.S. Pat. No. 4,104,796 to Sheldon which discloses radially shifted lugs which are spring loaded and shifted by centrifugal force out of engagement with locking lugs. The spring loaded lugs require special assembly and reduce the durability of the device. The patent to Sheldon also discloses the use of spring clips which engage and disengage notches on a perimeter of the spool carrier. The use of such spring clips is not preferred due to problems in manufacture, problems relating to the durability of spring elements and long term dependability.
The above problems encountered by the prior art are addressed by the invention which is summarized below.